1. Field of the Invention
The invention relates generally to the field of marine seismic surveying. More particularly, the invention relates to structures for marine seismic streamer systems and method for making such streamer systems that have improved noise suppression characteristics.
2. Background Art
In seismic surveying of the Earth's subsurface, data are obtained by applying seismic energy to the Earth near the surface and detecting seismic energy reflected from interfaces between different layers in subsurface formations. The seismic energy is reflected when there is a difference in acoustic impedance between the layer above the interface and the layer below the interface.
In marine seismic exploration, a seismic energy source, such as an air gun or array of air guns, for example, is used to generate seismic pulses in a body of water. The resulting seismic pulses are reflected back from subsurface interfaces and detected by sensors deployed in the water or on the water bottom. In a typical marine seismic operation, one or more cables called “streamers” are towed behind a vessel, typically at a water depth between about six to about nine meters. Sensors, typically hydrophones, are included in the streamers for detecting seismic signals. A hydrophone is a submersible pressure gradient sensor that converts the reflected seismic energy pressure waves into electrical or optical signals that are typically recorded for signal processing, and evaluated to estimate characteristics of the Earth's subsurface.
After the reflected seismic pulses reach the streamers, the reflected pulses continue to propagate to the water/air interface at the water surface, from which the pulses are reflected downwardly, and are again detected by the hydrophones in the streamers. The reflection coefficient at the surface is nearly unity in magnitude and negative in sign. The seismic pulses will thus be phase-shifted 180 degrees on reflection at the water surface. The downwardly traveling pulses are commonly referred to as the “ghost” signal, and the presence of this ghost signal creates a spectral notch in the seismic signal from the subsurface as detected by the hydrophones. Because of the spectral notch, some frequencies in the detected seismic signal are attenuated, whereas other frequencies are amplified.
Because of the ghost signal, the water surface acts like a filter, making it difficult to record seismic data outside a selected bandwidth without excessive attenuation or notches in the frequency spectrum of the recorded seismic data. Maximum attenuation will occur at frequencies for which the distance between the detecting hydrophone and the water surface is equal to one-half the wavelength of the seismic energy. Maximum amplification will occur at frequencies for which the distance between the detecting hydrophone and the water surface is one-quarter wavelength of the seismic energy. The wavelength of the seismic energy is equal to the velocity divided by the frequency, and the velocity of an acoustic wave in water is about 1500 meters per second. Accordingly the location in the frequency spectrum of the resulting spectral notch is readily determinable. For example, for a streamer water depth of 7 meters, maximum attenuation will occur at a frequency of about 107 Hz. and maximum amplification will occur at a frequency of about 54 Hz.
In “ocean bottom” seismic operations, in which sensors are deployed on the water bottom, it is well known to utilize particle motion sensors (typically geophones) in conjunction with pressure gradient sensors. A geophone detects energy in the form of particle velocity and generates a corresponding signal, whereas a hydrophone detects a pressure gradient and generates a corresponding signal. As stated above, the reflection coefficient at the surface is nearly unity in magnitude and negative in sign. The seismic pulses will thus be phase-shifted 180 degrees on reflection at the water surface. Further, the geophone has directional sensitivity, whereas the hydrophone does not. Accordingly, the upgoing wavefield signals detected by the geophone and the hydrophone will be in phase. The downgoing signal detected by the hydrophone and geophone is phase shifted by 180 degrees, but because the geophone is directionally sensitive, whereas the hydrophone is not, the downgoing wavefield signals detected by the geophone and the hydrophone will be 180 degrees out of phase. Various techniques have been proposed for using this phase difference to reduce the spectral notch caused by the ghost reflection. See for example, U.S. Pat. No. 4,486,865 to Ruehle; U.S. Pat. No. 5,621,700 to Moldoveanu; U.S. Pat. No. 4,935,903 to Sanders et al.; and U.S. Pat. No. 4,979,150 to Barr.
There have been various proposals for including particle motion sensors in streamer cables. See, for example, U.S. Pat. No. 7,239,577 which issued to Tenghamn et al. on Jul. 3, 2007. The main purpose of the particle motion sensors (typically geophones) is to provide data in the frequency spectrum around the ghost “notch” frequency, and to enable the determination of the upgoing and downgoing seismic wavefields. Such determination enables the streamer to be towed at greater depths without spectral notches in the seismic data in the frequency range of interest. At greater depths, the environment is less noise and the quality of the seismic data is improved, thereby increasing the “weather window” in which quality seismic data may be recorded.
In ocean bottom seismic operations, the particle motion sensor, typically a geophone, is placed in direct contact with the ocean bottom, and to improve the contact between the geophone and the ocean floor, the geophone assembly is typically made to be quite heavy. However, in order to include geophones in a steamer cable, the geophones need to be small, and the streamer cable motion will subject the geophone to greater noise than a geophone will experience when resting on the ocean floor.
Seismic steamer cables that include velocity sensors, and methods for reducing noise in the resulting signal resulting from geophone noise have been proposed. See, for example U.S. Pat. No. 7,239,577 which issued to Tenghamn et al. on Jul. 3, 2007 and US Published Application No. 2005/0195686, published on Sep. 8, 2005. However, there is a continuing need for systems and methods for noise suppression in the seismic signal detected by sensors in marine seismic streamers.